This invention relates to an optical bistable element comprising, between a pair of reflecting films operable as an etalon for an optical beam propagated between the reflecting films, a semiconductor layer having a refractive index variable with an intensity of the optical beam as a result of absorption by excitons. This invention relates also to a method of manufacturing such an optical bistable element.
An optical bistable element of the type described, will herein be referred to briefly as a semiconductor bistable etalon because the reflecting films serve like the known partially silvered glass plates of a conventional etalon. Such semiconductor bistable etalons are already known. Having optical switching and optical memory capabilities, the semiconductor bistable etalon is believed to be an important element in optical communication networks and optical information processing systems.
Semiconductor bistable etalons, such as a semiconductor superlattice etalon and a bulk semiconductor etalon, and a method of manufacturing a semiconductor superlattice etalon are described in a letter contributed by H. M. Gibbs et al to Applied Physics Letters, Volume 43, No. 3 (1 Aug. 1982), pages 221 to 222, under the title of "Room-temperature Excitonic Optical Bistability in a GaAs-GaAlAs Superlattice Etalon." According to the Gibbs et al letter, the semiconductor superlattice etalon comprises a GaAs-GaAlAs superlattice layer between a pair of dielectric coatings which serve as a pair of reflecting films of the etalon. The superlattice layer comprises sixty-one periods, each consisting of a 336-.ANG. GaAs layer and a 401-.ANG. Ga.sub.0.73 Al.sub.0.27 As layer, and has saturated exciton absorption in a multiple quantum well at room temperature.
When directed to one of the dielectric coatings, an incident optical beam is propagated through the superlattice layer as a propagated optical beam. If the incident optical beam has a certain input intensity or power, the propagated optical beam comes out of the etalon through the other dielectric coating as an excit or transmitted optical beam of an appreciable output intensity.
For an incident optical beam of a variable input intensity, the etalon shows excitonic optical bistability, namely, a refractive index response or characteristic which varies with the input intensity as a result of absorption of the propagated beam by excitons generated by the propagated beam. More particularly, the etalon shows a hysteresis loop in an input intensity versus output intensity characteristic. Incidentally, a columnar portion of such a semiconductor layer serves as an optical guide for the propagated optical beam.
According to Gibbs et al, the semiconductor superlattice etalon was manufactured as follows by molecular beam epitaxy (MBE). A layer of GaAs was grown on a silicon-doped GaAs substrate with an etch-stop layer of Ga.sub.0.73 Al.sub.0.27 As added. Sixty-one periods were grown with each GaAs layer followed by a Ga.sub.0.73 Al.sub.0.27 As layer. After polished down to less than 100 microns, the substrate was selectively etched for provision of a window which was 2 mm in diameter. Subsequently, dielectric coatings were evaporated on both surfaces to increase the reflectivity to nearly 90.degree./o between 820 and 890 nm.
The semiconductor superlattice etalon of Gibbs et al has a large refractive index nonlinearity at room temperature, is compatible with wavelengths of the present-day semiconductor laser diodes, and is adapted to high-density integration. Moreover, the etalon is operable at a high switch-on speed. However, the etalon has a low switch-off speed.
Semiconductor bistable etalons and a method of manufacturing a semiconductor superlattice etalon are described also in a letter contributed by Y. Silberberg et al to Applied Physics Letters, Volume 46, No. 8 (15 Apr. 1985), pages 701 to 703, under the title of "Fast Nonlinear Optical Response from Proton-bombarded Multiple Quantum Well Structures." It is known prior to the Silberberg et al letter that the switch-on speed is governed by an energy relaxation time for the excitons (between generation of the excitons by the propagated optical beam and decomposition by optical phonons of the excitons into electron and hole pairs) and is very short (one picosecond or shorter) and that the switch-off speed is governed by a recovery time of free carriers produced by decomposition of the excitons and is considerably long (a few scores of nanoseconds, such as 30 nanoseconds, in the case of natural recombination).
According to Siberberg et al, a semiconductor superlattice etalon comprises a superlattice layer which consists of eighty periods of 102-.ANG. GaAs layers alternated with 101-.ANG. Ga.sub.0.71 Al.sub.0.29 As layers grown by molecular beam epitaxy on an etch-stop layer of Ga.sub.0.71 Al.sub.0.29 As on a GaAs substrate. Such superlattice layers were bombarded with different doses of 200-keV hydrogen ions (protons). After the superlattice layers on the respective substrates were annealed for ten minutes at 300.degree. C., each substrate was selectively etched. An antireflection coating was evaporated over each of the superlattice layers to eliminate Fabry-Perot interferences. The etalon showed a free carrier recovery time of 150 picoseconds with a 10.sup.13 /cm.sup.2 dose. Although the free carrier recovery speed was fast, a certain decrease was observed in the nonlinear refractive index response. When a dose of 10.sup.14 /cm.sup.2 was used, the free carrier recovery time was only 33 picoseconds. The nonlinear refractive index response has, however, almost vanished.
Review of such conventional semiconductor bistable etalons clearly indicates the necessity of reducing the free carrier recovery time without adversely affecting the nonlienear refractive index characteristic or response. Incidentally, it is understood that recombination centers of deep levels are introduced according to Silberberg et al by injection of the protons into the superlattice layer.